This invention relates to sensors of the type used to generate electrical signals in response to relative motion between two or more sensor elements.
Many types of sensing devices are known which are capable of generating electrical signals in response to the relative motion between two or more sensor elements. Examples of such devices are electromagnetic transducers, in which relative motion between an electrically conductive coil and a magnet produces electrical current signals; electret transducers in which relative motion between an electret member and another member produces an electrical current; piezoelectric transducers, triboelectric transducers and strain gauges. All electromagnetic transducers function in accordance with the principles enunciated by James Clerk Maxwell, usually expressed in the form of the well known Maxwell's Electromagnetic Equation: ##EQU1## where .mu..sub.0 is the permeability of air, .epsilon..sub.0 is the permittivity of air, .phi..sub.E is the electric field, .beta. is the magnetic field, and l is the length of a closed loop ohmic conductor. The quantity i is the conduction current flowing in the conductor l, while the quantity [.epsilon..sub.0 (d.phi..sub.E /dt)] is termed the displacement current. This equation illustrates the interrelationship between a magnetic field and two electrical quantities: viz., the conduction current and the displacement current; and shows that a changing electric field acts as a source for a magnetic field in exactly the same manner as the conduction current which corresponds to charges actually moving along an ohmic conductor. The displacement current has the dimensions of a real current even though actual charges are not transported along an ohmic conductor. Thus, a magnetic field may be established in two ways: Firstly, by a changing electric field (the displacement current term); and secondly, by a conduction current (the conduction current term).
In all known electromagnetic sensors, in which a changing magnetic field is used to generate the electrical current signal, only the conduction current is sensed, since the magnitude of the displacement current is negligible when compared to that of the conduction current. In all other transducers of the class described above, the current produced by the transducer is also a conduction current.
While transducers which employ the conduction current are generally quite useful, certain limitations inhere in any transducer of this type. Since such transducers require a closed loop ohmic circuit for the conductive current, any varying electromagnetic radiation in the vicinity of the closed loop conductive path (which is usually coupled to amplifying the measuring circuitry) produces spurious conductive current signals which are capable of masking the information conveyed in the conductive current signals generated by the transducer. Accordingly, great care is required to shield the conductive loop portion of such sensors against stray electromagnetic radiation. Such shields introduce complexity to the structure of the transducers themselves and also to the structure of the conductive paths used to couple the transducer to the amplifying and measuring circuitry. In addition, with the exception of the electromagnetic transducer noted above, conductive current sensors require the application of electrical power to the closed loop portion of the circuit in order for the device to be operable. While this requirement poses no constraint in many applications, there are other applications for which this requirement is extremely burdensome. For example, in applications requiring the installation of many sensors in a structure for the purpose of monitoring vibrations of the structure, either individual sources of electrical power must be installed at the site of each sensor or the structure must be wired to provide electrical power from a central source to the individual sensors. In an office high-rise building, for example, in which it is desired to install several sensors on each floor of the building to provide vibration information, this requirement is at best costly and at worst impossible for pre-existing structures. In aircraft sensing applications, this requirement is similarly highly undesirable.
In addition to the above-noted disadvantages, conductive current sensors which employ permanent magnets possess additional disadvantages. For example, the energy required to magnetize a permanent magnet is relatively high when compared to the energy required to polarize an electret substance. Further, in sensor applications requiring geometry which is tailored to the geometry of the structural member whose motion or vibration is to be sensed, it can be quite costly and difficult to provide the necessary geometry for the permanent magnets. In addition, permanent magnets possess aging characteristics which result in a reduction of the magnetic field strength provided by the magnet with time, which requires recalibration of the system composed of the individual sensors at predetermined intervals and eventual replacement of the magnets.